The latest indictment charges Loughner with first-degree murder of a federal employee in the deaths of a federal judge and an aid to U.S. Congresswoman Gabrielle Giffords, gravely wounded in the attack at a meet-and-greet event for her constituents.

Loughner, 22, also is charged with causing the deaths of four other people who were "participants at a federally provided activity" when they were gunned down at the January 8 event.

Those charges, as well as additional counts of first-degree murder through the use of a firearm, are all capital offenses, and their inclusion starts a formal review process at the U.S. Department of Justice of deciding whether to seek the death penalty or life in prison for Loughner.

"This indictment involves potential death-penalty charges, and department rules require us to pursue a deliberate and thorough process," Arizona U.S. Attorney Dennis Burke said in a statement.

Loughner, a college dropout who investigators said had a history of mental problems, is accused of opening fire on Giffords and a crowd of bystanders attending a "Congress on Your Corner" event outside a grocery store.

Six people were killed and 13 others wounded, including Giffords, who was shot through the head at close range and remains hospitalized at a rehabilitation center in Houston.